Method for separating and treating exhaust gas of carbon fiber

ABSTRACT

The present invention provides a method for producing carbon fiber including the following processes: a process for obtaining fine carbon fiber by thermally decomposing an organic compound in a furnace by use of a catalyst; a process for separating a reaction exhaust gas contained in the carbon fiber; a process for continuously subjecting the carbon fiber to thermal treatment in a non-oxidative atmosphere; and a process for incinerating a thermal treatment exhaust gas generated in the thermal treatment and/or the reaction exhaust gas.  
     In connection with the production of carbon fiber, the present invention also provides a method for separating a reaction exhaust gas of carbon fiber, characterized in that a packed carbon fiber layer is formed, an inert gas is caused to flow through the layer, and the layer is compressed; and an apparatus for separating a reaction exhaust gas of carbon fiber, containing a compression chamber  3  having a compression cylinder  2  and an inert gas inlet  7,  which chamber is provided at the lower side of a separation tank  1  in which a packed layer of carbon fiber is formed, and a shut-off valve  4  which enables switching between compression and discharge.  
     The present invention contemplates combustion of the reaction exhaust gas and combustion exhaust gas generated from the subsequent thermal treatment, through employment of a pilot burner holding flame at all times in a vertical incinerator. Processing of exhaust gasses can be performed with safety under measures against fire by use of a flame detector and purge of the reaction exhaust gasses when reaction stops. These combustion methods and apparatuses.

CROSS REFERENCE TO RELATED APPLICATION

[0001] The present application is filed under 35 U.S.C. §111(a), andclaims benefit, pursuant to 35 U.S.C. §119(e)(1), of the filing date ofProvisional Application No. 60/163,391 filed Nov. 4, 1999 pursuant to 35U.S.C. §111(b).

FIELD OF THE INVENTION

[0002] The present invention relates to a method for separating areaction exhaust gas from production of carbon fiber and an apparatusused for separation of the gas, which exhaust gas is generated in areaction furnace during production of carbon fiber, more particularlyvapor-grown carbon fiber formed by thermal decomposition of an organiccompound in a reducing atmosphere containing hydrogen; to a method fortreating exhaust gasses and an incinerator used for treatment of thegasses, which gasses include the reaction exhaust gas and/or a thermaltreatment exhaust gas generated during thermal treatment, includingfiring and graphitization, performed in a post-process; and to a methodfor producing the carbon fiber.

BACKGROUND OF THE INVENTION

[0003] Carbon fiber is produced from a variety of raw materials, andfine carbon fiber is produced through a method in which an organiccompound such as methane, ethylene, benzene, or toluene is thermallydecomposed at 800-1,300° C. in a thermal-decomposition furnacecontaining a reducing gas such as hydrogen or carbon monoxide, by use ofa transition metal such as iron serving as a catalyst; i.e., a seed.

[0004] Specific examples of methods for producing carbon fiber include:

[0005] (1) a method for producing carbon fiber in which super-finepowder of a transition metal is distributed on a substrate in athermal-decomposition furnace and used as seeds (Japanese PatentApplication Laid-Open (kokai) No. 103528/1977);

[0006] (2) a method for producing carbon fiber in which a transitionmetal compound such as ferrocene is vaporized and introduced into athermal-decomposition furnace to thereby form super-fine powder of atransition metal, and the powder is used as seeds (Japanese PatentApplication Laid-Open (kokai) No. 54998/1985);

[0007] (3) a method for producing carbon fiber in which a transitionmetal such as iron is directly vaporized in a thermal-decompositionfurnace to thereby form super-fine powder, and the powder is used asseeds (Japanese Patent Application Laid-Open (kokai) No. 129986/1985);and

[0008] (4) a method for producing carbon fiber in which a transitionmetal compound such as ferrocene is diffused or dissolved in an organiccompound serving as a raw material, and the resultant mixture isintroduced into a thermal-decomposition furnace to thereby formsuper-fine powder of a transition metal, and the powder is used as seeds(Japanese Patent Application Laid-Open (kokai) No. 180615/1983).

[0009] Japanese Patent No. 2778434 discloses a method for producingcarbon fiber, in which an organic compound containing a transition metalsuch as iron, serving as a catalyst, is dissolved in a raw materialliquid such as benzene, and the resultant solution is sprayed on theinner wall of a reaction furnace heated at 800-1,300° C., to therebythermally decompose the material. Specifically, a transition metalcompound such as ferrocene, serving as a catalyst, is dissolved in aliquid organic compound such as benzene, and the resultant solution issprayed on the inner wall of a reaction tube, serving as athermal-decomposition furnace, by use of hydrogen serving as a carriergas, to thereby form seeds and thermally decompose the organic compound.As a result, crude carbon fiber of fine fibrous shape is produced.(Hereinafter the above process will be referred to as “the firstprocess.”)

[0010] The thus-produced carbon fiber or the reaction furnace containsflammable gases including a carrier gas such as hydrogen, andhydrocarbon generated in a side reaction (hereinafter the flammablegases will be collectively referred to as “reaction exhaust gas”), andthus the gas must be separated. A reaction exhaust gas which isseparated from carbon fiber in a reaction furnace is collected withrelative ease, but a reaction exhaust gas contained in carbon fiber, orin other words, captured between filaments of the carbon fiber, isdifficult to separate.

[0011] Conventionally, a reaction exhaust gas is separated from carbonfiber containing the reaction exhaust gas by means of the followingmethods: (1) a method in which the temperature of athermal-decomposition furnace is lowered after completion of reaction,and the inside of the furnace is substituted by nitrogen gas, to therebyseparate the reaction exhaust gas; and (2) a method in which a recoverycan is provided in a lower portion of a thermal-decomposition furnace,and carbon fiber containing a reaction exhaust gas is recovered in thecan and the inside of the can is substituted by nitrogen gas, to therebyseparate the exhaust gas.

[0012] However, when carbon fiber is industrially produced, in the abovemethod (1), reaction or recovery is carried out batchwise, which isdisadvantageous in terms of efficiency. In addition, the temperature ofa thermal-decomposition furnace must be lowered, which is unsatisfactoryin consideration of energy efficiency.

[0013] In the above method (2), a large recovery tube is required, dueto low bulk density of carbon fiber, which results in high cost.

[0014] In the methods (1) and (2), the produced carbon fiber has a verylow bulk density of 0.001-0.005 g/cm³ as measured immediately afterproduction, which means a large volume of space between fibers. Thus,gas held in such space cannot be completely removed from the carbonfiber, and may directly accompany the fiber product.

[0015] In addition, the carbon fiber is detrimentally difficult tohandle due to its low bulk density.

[0016] A recovered reaction exhaust gas is flammable and explosive,since the gas predominantly contains hydrogen. Therefore,conventionally, the gas is diluted in a blower in order to reduce theconcentration of hydrogen below the range causing explosion, and thenreleased in the air.

[0017] The crude carbon fiber produced in the reaction tube in the firstprocess is usually scraped off and collected. The collected carbon fibercontains non-reacted organic substances, non-fibrous carbides, and tar,and therefore, in the next process the carbon fiber is thermally treatedin a non-oxidative atmosphere. For example, the carbon fiber issubjected to thermal treatment such as firing and graphitization in aclosed furnace as disclosed in Japanese Patent Application Laid-Open(kokai) No. 60444/1996, in a non-oxidative atmosphere of nitrogen,helium, or argon at a temperature which varies depending on requiredproperties of a final product. (Hereinafter the above process will bereferred to as “the second process.”)

[0018] An exhaust gas generated in the second process predominantlycontains inert gasses such as argon and nitrogen. In addition, theexhaust gas contains naphthalene, anthracene, and high-molecular weightsubstances such as tar, and thus the gas is difficult to combust.(Hereinafter the gas will be referred to as “thermal treatment exhaustgas.”)

[0019] Since the thermal treatment exhaust gas is difficult to combust,there was no other way than releasing it as is.

[0020] The present invention contemplates provision of a method andapparatus for continuously separating a reaction exhaust gas from carbonfiber with ease and in a safe manner, which exhaust gas is generated inthe first process during production of carbon fiber through theabove-described vapor-growth method, as well as a method and apparatusfor combusting and air-releasing exhaust gasses at low cost, whichgasses include the flammable reaction exhaust gas and a thermaltreatment exhaust gas which is generated during thermal treatment in thesecond process and is difficult to combust.

[0021] The present invention also contemplates provision of a method forproducing carbon fiber, including the above methods and apparatus.

[0022] Particularly, exposure of an operator to an organic compound suchas benzene is regulated by the Law on Industrial Safety and Hygiene. Inaddition, such an organic compound is poisonous, and thus must beprevented from being released in the air. Meanwhile, hydrogen, methane,and ethylene are flammable substances, and leakage thereof may causeexplosion.

[0023] Tar is difficult to collect, because of its high viscosity. Amethod for condensing tar by use of activated carbon or for causing tarto be adsorbed by activated carbon requires large-scale handlingequipment, and tar poses problems in relation to waste treatment.

[0024] Moreover, in consideration of hygiene, tar must be handledcarefully. Tar is preferably incinerated for disposal, but tar per secannot be incinerated, since it is a high-molecular weight substance andcontains inert gasses.

[0025] In order to solve these problems, the present inventors havestudied a method for combusting a reaction exhaust gas. However, areaction in the first process needs to be terminated in order to carryout equipment maintenance, and at such times only the second process maybe carried out. In thermal treatment in the second process, the rate ofgeneration of a thermal treatment exhaust gas is not constant, since theamount of carbon fiber varies. Therefore, by means of only a method forcombusting such exhaust gasses, stable incineration is not carried out,and backfire to a reactor and a thermal treatment apparatus occurs,permitting damage to, for example, the apparatus. In addition, when fireis caused to be extinguished for some reason, a combustion apparatus maybe filled with a reaction exhaust gas of high concentration, whichprovides a problem at re-ignition.

SUMMARY OF THE INVENTION

[0026] Accordingly, the present invention provides a series of processesfor producing fine carbon fiber. Namely, the present invention providesa method for producing carbon fiber comprising: thermally decomposing anorganic compound in a thermal-decomposition furnace at 800-1,300° C. inan atmosphere containing a reducing gas, by use of a transition metal ora compound thereof serving as a catalyst, to thereby obtain fine carbonfiber; separating a reaction exhaust gas contained in the carbon fiberfrom the carbon fiber; continuously subjecting the carbon fiber tothermal treatment such as firing or graphitization in a non-oxidativeatmosphere; and incinerating a thermal treatment exhaust gas generatedduring the thermal treatment, and/or the separated reaction exhaust gas.

[0027] The present invention also provides a method and apparatus fortreating exhaust gasses, which gasses include an unwanted exhaust gaswhich is generated during a process for producing carbon fiber and anexhaust gas generated during thermal treatment such as firing orgraphitization in the second process.

[0028] Specifically, such a method and apparatus for treating exhaustgasses according to the present invention contemplate a first method andapparatus for industrially separating a flammable exhaust gas fromcarbon fiber predominantly containing the gas, in a safe and continuousmanner, and a second method and apparatus for incinerating in adedicated incinerator the separated reaction exhaust gas and a thermaltreatment exhaust gas from the second process, such that the gasses areincinerated independently or in combination. According to the firstmethod; i.e., a method for industrially separating a flammable exhaustgas from carbon fiber during production of carbon fiber, a packed layerof carbon fiber is formed, the carbon fiber being produced at an outletside of a reaction furnace of vapor-grown carbon fiber in the firstprocess; an inert gas is caused to flow upward from the lower side ofthe packed layer; and the packed layer is compressed. According to thefirst apparatus used for separating the exhaust gas from vapor-growncarbon fiber, there are provided, as shown in FIG. 1, a separation tank1 in which a packed layer of vapor-grown carbon fiber is formed; acompression chamber 3 provided at the lower portion of the tank, thechamber comprising a compression cylinder 2 and an inert gas inlet 7;and a shut-off valve 4 which enables switching between compression andexhaust.

[0029] According to the second method; i.e., a method for incineratingin a dedicated incinerator the separated reaction exhaust gas and athermal treatment exhaust gas from the second process, there areprovided the following:

[0030] (1) a method for treating an exhaust gas comprising incineratingan exhaust gas generated in the production process of vapor-grown carbonfiber (hereinafter referred to as “exhaust gas of vapor-grown carbonfiber”) of vapor-grown carbon fiber in a dedicated incinerator; and

[0031] (2) a method for treating an exhaust gas comprising mixingexhaust gasses; i.e., a flammable reaction exhaust gas and a thermaltreatment exhaust gas which is difficult to combust and which isgenerated during firing or graphitization, and incinerating the gassessimultaneously, or incinerating either of the gasses in the incinerator.The above methods (1) and (2) are further defined as follows:

[0032] (3) a method for treating an exhaust gas of vapor-grown carbonfiber, in which the gas is combusted after being ignited by a flame of apilot burner which is maintained by use of a flammable gas. Inconsideration of safety, the second method is preferably drawn to:

[0033] (4) a method for treating an exhaust gas of vapor-grown carbonfiber, in which a reaction exhaust gas in a reaction exhaust gas supplypipe is purged into the incinerator and incinerated upon completion ofthermal reaction of vapor-grown carbon fiber, or in which a thermaltreatment exhaust gas in a thermal treatment exhaust gas supply pipe ispurged into the incinerator and incinerated upon completion of thermaltreatment of the fiber;

[0034] (5) a method for treating an exhaust gas of vapor-grown carbonfiber, in which backfire-preventing apparatuses are provided in thesupply pipes of a reaction exhaust gas and a thermal treatment exhaustgas leading into the incinerator, so as to enhance safety of aproduction apparatus; and

[0035] (6) a method for treating an exhaust gas of vapor-grown carbonfiber, in which flames of the pilot burner and a primary burner aremonitored at all times, and when the flames are caused to beextinguished, supply of a reaction exhaust gas is switched to arelease-to-air pipe, to thereby stop supply of the exhaust gases intothe incinerator and enhance safety of the incinerator.

[0036] In order to attain any one of the above-described methods (1)through (6), the following incinerator is provided:

[0037] (7) an incinerator for processing an exhaust gas of vapor-growncarbon fiber, which comprises a primary burner for supplying a reactionexhaust gas, an auxiliary burner for supplying a thermal treatmentexhaust gas, and a pilot burner for igniting the exhaust gasses by useof a flammable gas.

[0038] As used herein, the term “carbon fiber” refers toannual-ring-form carbon fiber of multi-layer structure having a diameterof 0.01-5 μm, which fiber is vapor-grown in a thermal reaction furnaceaccording to the first and second method for treating an exhaust gas.

BRIEF DESCRIPTION OF THE DRAWINGS

[0039]FIG. 1 is a cross-sectional schematic representation showing anembodiment of a reaction exhaust gas separating apparatus of the presentinvention.

[0040]FIG. 2 is a cross-sectional schematic representation showing acompression process of carbon fiber in a reaction exhaust gas separatingapparatus of the present invention.

[0041]FIG. 3 is a cross-sectional schematic representation showing anextrusion-recovery process of carbon fiber in a reaction exhaust gasseparating apparatus of the present invention.

[0042]FIG. 4 is a cross-sectional schematic representation showing anembodiment of an incinerator of the present invention, which is used forincinerating a reaction exhaust gas and a thermal treatment exhaust gas.

DETAILED DESCRIPTION OF THE INVENTION

[0043] The present invention will next be described in more detail.

[0044] A separation tank is provided at the outlet side of a reactionfurnace, and carbon fiber is collected in the tank to thereby form apacked layer of carbon fiber.

[0045] The height of the layer is preferably 200 mm or more. When theheight is less than 200 mm, the layer tends to be contaminated with gas.The bulk density of the layer preferably falls within a range of0.005-0.05 g/cm³ inclusive.

[0046] When the bulk density is less than 0.005 g/cm³, resistance of thelayer becomes low and the layer tends to be contaminated with gas. Incontrast, when the bulk density is more than 0.05 g/cm³, purge gasencounters difficulty in flowing through the layer uniformly, and thusgas in the layer is insufficiently substituted by the purge gas. Thebulk density of carbon fiber can be regulated by controlling reactionconditions of carbon fiber or by compaction during conveyance of carbonfiber from a reaction furnace to a separation tank.

[0047] Purge gas used in the present invention may be any inert gas,such as nitrogen gas or argon gas. Purge gas is caused to flow from oneside to the other side of a packed layer of carbon fiber. In most cases,the produced carbon fiber contains gas which is lighter than air, andthus purge gas is preferably caused to flow from the lower side to theupper side of the layer.

[0048] Purge gas is preferably introduced into a packed layer of carbonfiber from the lower side at a linear velocity of 0.5 cm/second or more.When the linear velocity is less than 0.5 cm/second, the layer may becontaminated with gas.

[0049] In addition, in order to remove gas present between the filamentsof carbon fiber and increase the bulk density of carbon fiber, a packedlayer of carbon fiber is compressed.

[0050] A packed layer of carbon fiber is preferably compressed so as tohave a volume which is about ½ or less that before compression.Subsequently, a shut-off valve is opened and the compressed layer isintermittently removed through the valve. Alternatively, the compressedlayer of carbon fiber may be continuously removed without use of ashut-off valve, by extrusion through an outlet having a squeezed headportion.

[0051] When a packed layer of carbon fiber is compressed so as to have avolume which is in excess of ½ that before compression, the layer has alarge volume of interfiber space. Thus, when the layer is continuouslyremoved, gas in the layer cannot be separated sufficiently, permittinggas to accompany the carbon fiber product.

[0052] Specifically, a pressure for compression is 0.1 kg/cm² or more,preferably 1.0 kg/cm² or more. When the pressure is less than 0.1kg/cm², gas held between filaments of the carbon fiber is insufficientlyseparated. The pressure may be higher, preferably to the extent thatcarbon fiber does not collapse. When the pressure becomes higher,facility cost of the pressurizing system becomes higher. Therefore, thepressure preferably falls within a range of 0.1-100 kg/cm², morepreferably 1-100 kg/cm², still more preferably 1-50 kg/cm².

[0053] The process by use of purge gas may be carried out prior to thecompression process, or vice versa. Alternatively, these processes maybe repeated alternately. However, in consideration of time andefficiency, the process making use of purge is carried out prior to thecompression process, to thereby enhance effects. A compressed carbonfiber obtained through these processes is preferable in consideration ofeasy handling to the next process.

[0054] The present invention will be described in more detail withreference to FIGS. 1, 2, 3, and 4, which description should not beconstrued as limiting the invention thereto.

[0055] In FIGS. 1, 2, and 3, reference numeral 1 represents a separationtank, 2 represents a compression cylinder, 3 represents a compressionchamber, 4 represents a shut-off valve, 5 represents a recovery vessel,6 represents an outlet of an apparatus for producing carbon fiber, 7represents an inert gas inlet, 8 represents an inert gas outlet, and 9represents the height of a packed layer of carbon fiber.

[0056] Gas-containing carbon fiber which is produced in a reactionfurnace (not shown in the Figs.) is conveyed to the separation tank 1through the outlet 6, to thereby form a packed layer of the carbon fiberin the tank 1.

[0057] In this case, the height of the layer 9 is maintained at 200 mmor more. As described above, when the height is less than 200 mm, thelayer tends to be contaminated with gas.

[0058] Subsequently, nitrogen gas is introduced through the inert gasinlet 7 provided in the compression chamber 3 at the lower side of thepacked layer, to thereby purge gas contained in the layer. The inlet(s)may be provided in a quantity according to the size or height of apacked layer of carbon fiber.

[0059] Next, as shown in FIG. 2, the carbon fiber is compressed by useof the compression cylinder 2 so as to have a volume which is ½ or lessthat before compression. The pressure for compression preferably fallswithin a range of 0.1-100 kg/cm². The carbon fiber may be removedwithout use of the shut-off valve 4, by extrusion through an outlethaving a squeezed head portion.

[0060] As shown in FIG. 3, the shut-off valve 4 is opened, and thethus-compressed carbon fiber is recovered in the recovery vessel 5.Alternatively, the carbon fiber is recovered in the vessel 5 byextrusion without use of the shut-off valve 4, through an outlet havinga squeezed head portion.

[0061] When the recovery vessel 5 is filled with the carbon fiber, thevessel is replaced by a new one, to thereby continuously carry outseparation of gas from carbon fiber.

[0062] In a method for treating a reaction exhaust gas and/or a thermaltreatment exhaust gas of vapor-grown carbon fiber according to thepresent invention, as shown in FIG. 4, a vertical incinerator 18 isemployed, a flammable gas is continuously combusted in the incinerator18 by use of a pilot burner 11, and a reaction exhaust gas is suppliedthrough a primary burner 12 into the incinerator and ignited by use of aflame of the pilot burner 11, to thereby incinerate the exhaust gas.

[0063] A flammable gas may be any of commercially available gasses, suchas propane gas or city gas.

[0064] A reaction exhaust gas in the present invention is usuallyflammable and capable of being incinerated. However, in the case ofincineration of the gas, since the reaction exhaust gas is flammable,the following must be carefully considered: a method for igniting thereaction exhaust gas when the gas is generated after initiation ofreaction; change in pressure of the reaction exhaust gas duringreaction; and occurrence of backfire when generation of the reactionexhaust gas stops after completion of reaction.

[0065] Therefore, in a method for igniting a reaction exhaust gas, thepilot burner 11 is turned on at all times in the incinerator 18 suchthat a reaction exhaust gas is ignited immediately after supply of thegas. The reaction exhaust gas is supplied through a reaction exhaust gassupply pipe 19 and the primary burner 12, which comprises a nozzleportion for providing the gas with resistance, and thus backfire doesnot occur even when changes in pressure occur.

[0066] When supply of a reaction exhaust gas is terminated aftercompletion of reaction, a reaction exhaust gas in the reaction exhaustgas supply pipe 19 is purged into the incinerator 18 by use of anincombustible gas, and the remaining reaction exhaust gas in a reactionsystem and the supply pipe 19 is incinerated, to thereby preventbackfire into the supply pipe 19.

[0067] Flame of the pilot burner 11 and a combustion flame of a reactionexhaust gas of the primary burner 12 are monitored at all times in theincinerator 18. When the flames are caused to be extinguished for somereason, the reaction exhaust gas cannot be ignited, and thus thereaction exhaust gas is supplied not to the incinerator 18 but to arelease-to-air pipe 21, and supply of the exhaust gas to the incinerator18 is terminated. In addition, the reaction is immediately terminatedand the reaction exhaust gas is purged by use of an incombustible gas.

[0068] A checking apparatus such as a flame arrester or a water sealingapparatus may be provided in the supply pipe 19 of a reaction exhaustgas into the incinerator 18.

[0069] Furthermore, sequence control may be performed so as to regulatea reaction in which a flammable reaction exhaust gas is generated, suchthat the reaction proceeds only when the pilot burner 11 in theincinerator 18 is turned on.

[0070] The present invention is characterized in that a supply pipe 20of a thermal treatment exhaust gas which is difficult to combust isindependently provided and the exhaust gas is supplied to theincinerator through a auxiliary burner 15. Accordingly, hydrogen;organic compounds such as methane and ethylene which have relativelyhigh volatility and high flammability; and tar and the like which havehigh viscosity at ambient temperature and are relatively difficult tocombust are combusted simultaneously. As a result, the mixture gasenables combustion of the thermal treatment exhaust gas, and combustionefficiency is enhanced.

[0071] The above-described ignition method, the backfire-preventingmethod, the purge method, and the treatment when fire is caused to beextinguished in a reaction exhaust gas are applicable to a thermaltreatment exhaust gas.

[0072] Even when supply of a reaction exhaust gas is terminated, since aflammable gas is used as a fuel, only a thermal treatment exhaust gascan be supplied to the incinerator and combusted efficiently. Inaddition, the pilot burner 11 in the incinerator is turned on at alltimes, and thus a reaction exhaust gas and a thermal treatment exhaustgas can be stably combusted regardless of change in amount thereof.

EXAMPLES Example 1

[0073] In a vertical reaction furnace, a benzene raw-material solutioncontaining 4 wt. % ferrocene as dissolved therein was sprayed onto theinner wall of a reaction tube through a two-fluid spray nozzle by use ofhydrogen gas serving as a carrier gas, to thereby grow carbon fiber. Theflow rate of hydrogen gas was 100 L/minute and the temperature of theinner wall was 1,200° C. The carbon fiber grown on the inner wall wasscraped off, to thereby obtain crude carbon fiber. The crude carbonfiber was collected in a separation tank 1 shown in FIG. 1.

[0074] When the height of a packed layer of the collected carbon fiberin the tank 1 became 200 mm, nitrogen gas was caused to flow through aninert gas inlet 7 at a rate of 50 L/minute. When the height of the layerin the tank 1 became 500 mm or more, a compression cylinder 2 wasoperated, and the vapor-grown carbon fiber was compressed so as to havea volume which was ½ that before compression. In this case, the pressurefor compression was 1 kg/cm².

[0075] The bulk density of the packed layer before compression was 0.01g/cm³.

[0076] Subsequently, a shut-off valve 4 was opened, and the compressioncylinder 2 was re-operated to thereby discharge the compressed carbonfiber into a recovery vessel 5. Thereafter, the cylinder 2 was moved toits original position, and the valve 4 was closed.

[0077] While the reaction of carbon fiber was carried out, theabove-described procedures was repeated, to thereby recover carbon fiberin the recovery vessel.

[0078] During recovery of carbon fiber, hydrogen in the recovery vesselwas analyzed through a hole for sampling, by use of a hydrogen detector(model: GP-226, product of Riken Keiki), but hydrogen was not detected.Methane gas and ethylene gas, which are generated during the reaction ofcarbon fiber, were analyzed through gas chromatography but not detected.

[0079] The recovered carbon fiber in the recovery vessel had a diameterof 0.06 μm and a bulk density of 0.02 g/cm³.

Example 2

[0080] The carbon fiber obtained in Example 1 was subjected tocompression molding, and thermally treated under flow of argon in acontinuous thermal treating furnace as disclosed in Japanese PatentApplication Laid-Open (kokai) No. 60444/1996. The molded product washeated in the furnace at 1,400° C. for approximately 30 minutes.

[0081] A reaction exhaust gas generated in the process of Example 1 wassupplied through a primary burner 12 at a rate of approximately 130L/minute, and a thermal treatment exhaust gas generated in the abovethermal treatment was supplied through an auxiliary burner 15 at a rateof approximately 30 L/minute. The reaction exhaust gas containedhydrogen in an amount of approximately 75%, benzene serving as a rawmaterial; i.e., a carbon source, and organic compounds which had beenby-produced. The thermal treatment exhaust gas contained argon as aprimary component, and tar.

[0082] Before the reaction in the reaction furnace and before thethermal treatment in the thermal treatment furnace, the system waspurged by use of nitrogen gas, and combustion was initiated. Thereaction exhaust gas was supplied through the primary burner 12 to anincinerator 18 in which LPG serving as a fuel was combusted by use of apilot burner 11, and the exhaust gas was combusted and incinerated.Simultaneously, the thermal treatment exhaust gas was supplied throughthe auxiliary burner 15 and combusted. The thus-incinerated waste gascontained benzene in an amount of 0.25 ppm or less, which represents thedetection threshold.

[0083] After completion of the first process, a reaction vessel systemwas purged by use of nitrogen gas and the reaction exhaust gas wasremoved, and a primary valve was closed.

[0084] After completion of all the above processes, the continuousfurnace system was purged by use of nitrogen gas, the thermal treatmentexhaust gas was purged by use of nitrogen gas, and a flame of theprimary burner 12 was extinguished.

[0085] In a method for treating an exhaust gas of vapor-grown carbonfiber of the present invention, a reaction exhaust gas containingmethane, ethylene, and a flammable carrier gas such as hydrogen which iscontained in carbon fiber during production thereof can be continuouslyseparated with safety. In addition, the reaction exhaust gas and athermal treatment exhaust gas generated in a thermal treatment processwhich is difficult to combust are completely combusted with safety.

1. A method of separating a reaction exhaust gas of carbon fiber duringproduction of carbon fiber, the production comprising thermallydecomposing an organic compound in a thermal-decomposition furnace at800-1,300° C. in an atmosphere containing a reducing gas, by use of atransition metal or a compound thereof serving as a catalyst,characterized in that the method includes forming a packed layer ofcarbon fiber at an outlet side of the thermal decomposition furnace,causing an inert gas to flow upward from the lower side of the packedlayer, and compressing the packed layer.
 2. The method of separating areaction exhaust gas of carbon fiber according to claim 1, wherein thecarbon fiber has a diameter of 0.01-5 μm.
 3. The method of separating areaction exhaust gas of carbon fiber according to claim 1 or 2, whereinthe packed layer has a height of 200 mm or more.
 4. The method ofseparating a reaction exhaust gas of carbon fiber according to claim 1or 2, wherein the inert gas as recited in claim 2 has a linear velocityof 0.5 cm/second or more.
 5. The method of separating a reaction exhaustgas of carbon fiber according to claim 3, wherein the packed layer iscompressed to have a volume which is ½ or less that before compression.6. The method of separating a reaction exhaust gas of carbon fiberaccording to claim 5, wherein the packed layer is compressed under apressure of 0.1-100 kg/cm².
 7. Apparatus for separating a reactionexhaust gas of carbon fiber, the apparatus being used for carrying out amethod as described in claim 1, which apparatus includes, at a lowerportion of a separation tank 1 in which a packed layer of vapor-growncarbon fiber is formed, a compression chamber 3 comprising a compressioncylinder 2 and an inert gas inlet 7, and a shut-off valve 4 whichenables switching between compression and exhaust.
 8. A method fortreating an exhaust gas generated from production of carbon fiber,characterized in that a reaction exhaust gas separated by use of amethod as recited in claim 1 is incinerated in an incinerator.
 9. Amethod for treating an exhaust gas of carbon fiber during production ofcarbon fiber, the production comprising thermally decomposing an organiccompound in a thermal-decomposition furnace at 800-1,300° C. in anatmosphere containing a reducing gas, by use of a transition metal or acompound thereof serving as a catalyst, characterized in that the methodincludes incinerating, in an incinerator, a reaction exhaust gas from aproduction process in the thermal-decomposition furnace and/or a thermaltreatment exhaust gas which is generated in a subsequent process. 10.The method for treating an exhaust gas of carbon fiber according toclaim 9, wherein the incinerator includes a pilot burner for ignitionand a burner for supplying a reaction exhaust gas.
 11. The method fortreating an exhaust gas of carbon fiber according to claim 9 or 10,wherein the carbon fiber has a diameter of 0.01-5 μm.
 12. The method fortreating an exhaust gas of carbon fiber according to claim 11, whereinthe reaction exhaust gas is flammable.
 13. The method for treating anexhaust gas of carbon fiber according to claim 11, wherein the reactionexhaust gas is difficult to combust.
 14. The method for treating anexhaust gas of vapor-grown carbon fiber according to claim 9 or 10,wherein ignition for combustion is effected by flame from the pilotburner through use of the flammable gas serving as a fuel.
 15. Themethod for treating an exhaust gas of vapor-grown carbon fiber accordingto claim 9 or 10, wherein the reaction exhaust gas in a reaction exhaustgas supply pipe is purged into the incinerator and incinerated uponcompletion of reaction of carbon fiber, or in which a thermal treatmentexhaust gas in a thermal treatment exhaust gas supply pipe is purgedinto the incinerator and incinerated upon completion of thermaltreatment of the carbon fiber.
 16. The method for treating an exhaustgas of carbon fiber according to claim 9 or 10, whereinbackfire-preventing apparatuses are provided in the supply pipes of thereaction exhaust gas and the thermal treatment exhaust gas leading intothe incinerator.
 17. The method for processing an exhaust gas of carbonfiber according to claim 9 or 10, wherein flames of the pilot burner anda primary burner are monitored at all times, and when the flames arecaused to be extinguished, supply of a reaction exhaust gas is switchedto a release-to-air pipe, to thereby stop supply of the exhaust gasesinto the incinerator.
 18. An incinerator for an exhaust gas of carbonfiber, which incinerator is used for production of carbon fibercomprising thermally decomposing an organic compound in athermal-decomposition furnace at 800-1,300° C. in an atmospherecontaining a reducing gas, by use of a transition metal or a compoundthereof serving as a catalyst, wherein the incinerator includes aprimary burner for supplying a reaction exhaust gas which is flammable,an auxiliary burner for supplying a thermal treatment exhaust gas, and apilot burner for igniting the exhaust gasses by use of a flammable gas.19. The incinerator for an exhaust gas of carbon fiber, wherein thecarbon fiber has a diameter of 0.01-5 μm.
 20. A method for producingcarbon fiber comprising a first step of thermally decomposing an organiccompound in a thermal-decomposition furnace at 800-1,300° C. in anatmosphere containing a reducing gas, by use of a transition metal or acompound thereof serving as a catalyst, to thereby obtain carbon fiber,and a second step of separating a reaction exhaust gas produced duringthe first step.
 21. The method for producing carbon fiber according toclaim 20, wherein the carbon fiber has a diameter of 0.01-5 μm.
 22. Amethod for producing carbon fiber comprising a first step of thermallydecomposing an organic compound in a thermal-decomposition furnace at800-1,300° C. in an atmosphere containing a reducing gas, by use of atransition metal or a compound thereof serving as a catalyst, to therebyobtain carbon fiber, and a second step in which the carbon fiber issubjected to a thermal treatment of firing or graphitization in anon-oxidative atmosphere.
 23. The method for producing carbon fiberaccording to claim 22, wherein the carbon fiber has a diameter of 0.01-5μm.
 24. A method for producing carbon fiber comprising a first step ofthermally decomposing an organic compound in a thermal-decompositionfurnace at 800-1,300° C. in an atmosphere containing a reducing gas, byuse of a transition metal or a compound thereof serving as a catalyst,to thereby obtain carbon fiber, a second step of separating a reactionexhaust gas produced during the first step, a third step in which thecarbon fiber is subjected to a thermal treatment of firing orgraphitization in a non-oxidative atmosphere, and a fourth step ofincinerating a thermal treatment exhaust gas from the thermal treatmentstep and/or the separated reaction exhaust gas.
 25. The method forproducing carbon fiber according to claim 24, wherein the carbon fiberhas a diameter of 0.01-5 μm.
 26. The method for producing vapor-growncarbon fiber according to claim 24 or 25, wherein the reaction exhaustgas contained in the carbon fiber is separated.
 27. The method forproducing carbon fiber according to claim 24, wherein incineration ofthe thermal treatment exhaust gas from the thermal treatment step and/orthe separated reaction exhaust gas is effected through ignition by useof flame from a pilot burner making use of a flammable gas as a fuel.28. The method for producing carbon fiber according to claim 24, whereinincineration of the thermal treatment exhaust gas from the thermaltreatment step and/or the separated reaction exhaust gas is effected insuch a manner that the reaction exhaust gas in a reaction exhaust gassupply pipe is purged into the incinerator and incinerated uponcompletion of reaction of carbon fiber, or that a thermal treatmentexhaust gas in a thermal treatment exhaust gas supply pipe is purgedinto the incinerator and incinerated upon completion of thermaltreatment of the carbon fiber.
 29. The method for producing carbon fiberaccording to claim 24, wherein incineration of the thermal treatmentexhaust gas from the thermal treatment step and/or the separatedreaction exhaust gas is effected in the presence of backfire-preventingapparatuses provided in the supply pipes of the reaction exhaust gas andthe thermal treatment exhaust gas leading into the incinerator.
 30. Themethod for producing carbon fiber according to claim 24, whereinincineration of the thermal treatment exhaust gas from the thermaltreatment step and/or the separated reaction exhaust gas is performedsuch that flames of a pilot burner and a primary burner are monitored atall times, and when the flames are caused to be extinguished, supply ofa reaction exhaust gas is switched to a release-to-air pipe, to therebystop supply of the exhaust gases into the incinerator.